Communication systems that utilize coded communication signals are known in the art. One such system is a direct sequence code division multiple access (DS-CDMA) cellular communication system, such as set forth in the Telecommunications Industry Association Interim Standard 95B (TIA IS-95B) herein after referred to as IS-95. In accordance with IS-95, the coded communication signals used in the DS-CDMA system comprise spread spectrum signals which are transmitted in a common 1.25 MHz bandwidth channel between mobile stations and base transceiver stations (BTS) located at base sites of the wireless communication system. Each 1.25 MHz bandwidth portion of the radio-frequency (RF) spectrum, or 1.25 MHz bandwidth channel, carries spread spectrum signals centered around a particular carrier frequency and is commonly referred to as a narrowband DS-CDMA channel. Recovery of the spread spectrum signals by the BTS is enabled via the use of non-coherent demodulation techniques.
In addition, there are numerous international standards proposals which suggest a coherent wideband CDMA transmission signal configuration. Evolution from the current narrowband CDMA signal transmission configuration, which is designed to carry voice and low rate data, has been driven by a need to deploy higher rate circuit and packet data services in a wireless communication environment. In order to accommodate the higher rate services, wideband CDMA signal transmission configurations have been proposed in which a coherent demodulation scheme is utilized. The coherent demodulation scheme is enabled via the use of a pilot signal added to the uplink signal path from the mobile station to the BTS. The pilot signal is used by a receiver multipath signal searcher in the BTS to enable differentiation between transmitted mobile station signals and unwanted noise interference.
A mobile communication signal transmitted from a mobile station to a BTS, may be reflected off of nearby scatterers, such as buildings, and result in multipath propagation of the transmitted signal. These reflections produce replicas, typically referred to as multipath replicas, of the originally transmitted signal which arrive at a base site receiver with various power levels at various times. The power levels are determined by propagation distances traveled by the multipath replicas. Upon receipt by the BTS, the originally transmitted signal and its multipath replicas are filtered, despread, recombined and decoded to yield the desired voice or data signal.
The ability of a spread spectrum system to resist noise interference, commonly referred to as narrowband interference, is determined in part by the processing gain of the system. Wideband CDMA system configurations inherently possess a higher processing gain (a ratio of occupied bandwidth to an information rate), when compared to narrowband CDMA systems. The higher processing gain has the advantage of increased noise immunity as well as higher signal resolution due to additional multipath components available in wideband CDMA systems.
Unfortunately, multipath component, or signal searching in a wideband wireless communication system is less effective when compared with multipath component searching in an narrowband (IS-95) wireless communication system. First, a high energy single signal typically resolved in an IS-95 wireless communication system becomes multiple low energy signals, resolved in a wideband wireless communication system. Hence, a wideband wireless communication system has a lower multipath signal detection probability due to less available signal energy per multipath signal. Second, in a wideband wireless communication system, a reduced chip period for a given search window yields a larger number of pn offsets to be searched. Consequently, the probability that a pn-offset within the search window may be erroneously detected is increased, thereby yielding a higher overall probability of false alarms. Accordingly, a multipath searcher with substantially more sensitivity is required in a wideband wireless communication system in order to overcome the problems associated with lower multipath signal detection and higher false alarms in the BTS.
Therefore, a need exists for a method and apparatus to provide a signal search capability within a wideband wireless communication system that is easy to implement and overcomes low multipath signal detection and increased false alarm problems inherent in the prior art design.